In a conventional radio communication system performing high-speed packet transmission, a downlink that transmits a signal from a base station to a radio communication apparatus, and an uplink that transmits a signal from a radio communication apparatus to a base station, are divided, and are estimated by measuring the quality of the respective transmission paths (hereinafter referred to as “channel quality”). Based on this estimated channel quality, the base station performs scheduling that allocates frequency and time resources to an accessed radio communication apparatus, and also sets transmission power and transmission speed (an M-ary value and a coding rate) and transmits data.
In this kind of radio communication system, there are three methods of dividing an uplink and downlink: a TDD (Time Division Duplex) method, FDD (Frequency Division Duplex) method, and CDD (Code Division Duplex) method.
Of these, an FDD method divides an uplink and downlink by frequency, and uses different frequencies in an uplink and downlink.
In recent years, in the radio communication field, data communication in which the amount of information on a downlink greatly exceeds that on an uplink has been predicted to become the mainstream, and the development of asymmetric-communication radio communication systems in which the frequency band of a downlink is wider than that of an uplink has been pursued.
The introduction of a wideband system with a bandwidth even greater than that used in a 3GPP mobile communication system has been studied as a fourth-generation mobile communication system (IMT-advanced). With communication channels being made wideband, frequency selectivity can no longer be ignored.
Thus, in a system using a wider-band communication channel than a 3GPP mobile communication system, the introduction of frequency scheduling whereby a frequency at which transmission path conditions are comparatively good is detected for each radio communication apparatus and a frequency is allocated, and adaptive modulation using a modulation method and coding rate that satisfy a predetermined packet error rate according to transmission path conditions at an allocated frequency, has been studied.
Specifically, the use of multicarrier transmission such as OFDM (Orthogonal Frequency Division Multiplexing), OFDMA (Orthogonal Frequency Division Multiple Access), or MC-CDMA (Multi-Carrier Code Division Multiple Access), has been considered as an IMT-advanced transmission method.
In these multicarrier transmission methods, high-speed transmission is implemented by using many subcarriers. Also, SC-FDMA (Single Carrier-Frequency Division Multiple Access) has been studied as a transmitting method for an uplink from a radio communication apparatus to a base station.
This SC-FDMA is a transmitting method whereby a signal resulting from converting a single-carrier-modulated signal to the frequency domain is placed on a specific carrier and used as a subcarrier signal in OFDM, as disclosed in Non-patent Document 1, for example.
Subcarrier placement in SC-FDMA may be localized placement whereby placement is performed densely in a specific frequency block, or distributed placement whereby placement is performed at specific subcarrier intervals. Use of these types of placement enables the PAPR (Peak to Average Power Ratio) to be reduced.
This characteristic of enabling the PAPR to be reduced makes SC-FDMA a particularly suitable uplink transmission method for a battery-operated radio communication apparatus for which power consumption is a concern.
Studies have been carried out into performing adaptive modulation and frequency scheduling on a subcarrier-by-subcarrier basis, or in units of subcarrier blocks each composed of a plurality of subcarriers, using these transmission methods.
In a system that performs this kind of adaptive modulation and frequency scheduling, it is necessary for a radio communication apparatus to report instantaneous individual subcarrier or subcarrier block unit channel quality information (CQI: Channel Quality Indicator) to a base station in a downlink.
On the other hand, in an uplink, it is necessary for a radio communication apparatus to perform channel quality measurement pilot signal transmission to enable the base station to measure instantaneous individual subcarrier or subcarrier block unit channel quality information (CQI) (hereinafter referred to as “uplink channel quality measurement”).
Uplink channel quality measurement can be performed using channel duality in the case of a TDD (Time Division Duplex) system, and using a downlink CQI report as uplink channel quality when channel fluctuation is sufficiently small.
However, in the case of a cellular system, adaptive modulation and frequency scheduling are necessary that take account of the fact that the amount of interference is different in an uplink and a downlink.
While a data demodulation pilot may be transmitted in the same band as data, for an uplink channel quality measurement pilot signal it is necessary to perform transmission using subcarriers of all bands (conventional method 1) or to perform transmission across at least sufficiently more subcarriers than the frequency resources to be allocated using some bands within the bands (partial bands) (conventional method 2). In the case of conventional method 2, a subcarrier for which transmission path conditions are better can be detected by performing multiple transmissions of different partial bands. Also, it is necessary for a channel quality measurement pilot signal such as described above to be transmitted irrespective of the presence or absence of data.    Non-patent Document 1: Ofuji, Kawamura, Higuchi, Sawahashi, “Frequency Domain Channel-Dependent Scheduling with Group-wised Allocation of Transmission Bandwidth of Pilot Channel for CQI Measurement in Single-Carrier FDMA-Based Evolved UTRA Uplink”, Technical Report of IEICE, RCS2006-154, October, 2006